Smoke

Smoke makes light visible. A beam that was doing its job invisibly becomes an object in the frame, with an edge and a direction, and the room gains a shape it did not have.

It also moves. Haze sits still, and smoke rolls and curls, so it puts slow motion into a locked frame and gives a held shot something to do.

It needs a hard source. A tight beam through smoke reads as a solid shaft. A soft source through the same smoke reads as fog, because there is no edge for the particles to describe.

Density and movement are separate controls. A thin roll drifting past the lens is a mood. A thick bank is an obstruction. Say which one you want and say how fast it travels.

Backlight and side light do the work. Smoke lit from the front goes grey and dead, and the beam disappears with it.

Use it when the light itself should be part of the picture, and in any interior where a window or a lamp has to throw a visible shaft.

Avoid it when the background has to stay readable for the whole take, because a roll of smoke will cross the frame and take part of it away.

Haze is even, still and thin, and it lowers contrast with distance. Smoke is uneven, moving and local. The test is whether the air has a shape, because haze has none.

Steam rises in columns from a heat source. Smoke rolls and spreads in any direction. If it goes up in a line, it is steam.

Ridley Scott fills the corridors of Alien (1979) and the streets of Blade Runner (1982) with smoke, so every practical light throws a visible beam. The light does most of the set dressing in both, because the smoke is what gives it a body.

The smoke arrives with nothing to catch. Without a hard source in the scene the particles have no beam to describe and the result is grey air, so name the light and say it is hard.

The second failure is smoke that hangs still. Ask for the roll and the curl, and give it a direction across the frame.